1. Field of the Invention
The present invention relates to a photothermographic material preferably used in the field of films for medical diagnosis, the field of films for graphic arts, or the like.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Photothermographic materials utilizing organic silver salts have been made known by “Thermally Processed Silver Systems” by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. In general, photothermographic materials have an image forming layer in which a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region.
Japanese Patent Application Laid-Open (JP-A) Nos. 10-10670 and 10-62899 disclose a technique for preparing an image forming layer using a polymer latex as a binder and an aqueous medium. JP-A No. 2002-303953 discloses a technique for using a polymer latex having a specific physical character as a binder to improve manufacturing-related brittleness and image storability under dark storage conditions (fogging during storage) of photothermographic materials. JP-A No. 11-84573 discloses a technique for using a specific polymer latex as a binder for the image forming layer and a surface protective layer to attain low fog and high Dmax. Further, JP-A Nos. 11-119375 and 11-288058 disclose a technique for providing an intermediate layer having a water-soluble polymer or a polymer latex, between an image forming layer and a surface protective layer.
However, photothermographic materials contain all components necessary for forming an image in a film thereof in advance, and unreacted components and reaction products remain in the film after the image is formed. The unreacted components and the reaction products have a big influence on raw stock storability and image storage stability, and the techniques described above have not provided sufficient improvement. Thus, further improvements are demanded.